The present invention is an improvement over the processes described in U.S. Pat. No. 3,919,393 and U.S. Pat. No. 3,919,394, the contents of both said patents being incorporated by reference herein, and also an improvement over my copending application entitled "Sulfur Dioxide Removal from Gaseous Streams," Ser. No. 865,513 filed Dec. 29, 1977, the contents of said application also being incorporated by reference herein.
The operation of lime scrubbing processes for the removal of oxides of sulfur from the flue gas stream is generally a continuous process so that a power plant can at all times maintain operation within the boundaries of the environmental laws. There does occur, however, periods of time when a particular scrubbing unit is shutdown, either intentionally or due to malfunction of apparatus or other undesired shutdowns. For example, in a large power plant, a series of scrubbing units may be provided, all of which are operational during periods of peak load upon the power plant, but during periods of lesser load upon the plant capacity, operation of one or more of the scrubbing units may be unnecessary and the system could still effect scrubbing of the flue gases within the prescribed limits.
During the shutdown of a scrubbing unit, or other interruption of flow of scrubbing liquor through the scrubber, a problem arises in that the reactive species in the liquor, which are usually retained in a hold tank during the shutdown, are subject to oxidation due to contact with air. Such liquors will contain solids and, while in the hold tank, are stirred so as to prevent settling and possible plugging of the hold tank by such settling. Even where the liquor is not held but recirculation of the liquor through the scrubbing unit is maintained, the exposure of the reactive species in the liquor to air during the recirculation will cause oxidation of the reactive species. These reactive species include sulfite and bisulfite ions which, upon oxidation, will be converted to sulfate.
Depending upon the length of the shutdown or interruption of flow of the gaseous stream through the scrubber and the amount of oxygen absorbed by the scrubbing liquor, a considerable portion of the reactive sulfite ions can be oxidized to sulfate and the scrubbing liquor loses the reactive sulfite species so that the liquor will be less able to absorb the oxides of sulfur upon resumption of flow of the scrubbing liquor. Also, the oxidation of the sulfite ions to sulfates produces a super-saturated system for sulfates and is conducive to formation of gypsum which produces scaling in the system.
With the oxidation of the sulfite ions to sulfate, the scrubbing unit upon resumption of operation will not operate at the required sulfur dioxide and sulfur trioxide removal efficiencies required by the environmental laws and may be operating outside the prescribed limits for a period of six hours or more before reaching the desired operational mode.
The present method is directed to a process for treating the scrubbing liquor upon interruption of flow of the gaseous stream through the scrubbing unit so that the liquor will be capable of absorbing oxides of sulfur upon resumption of flow of oxides of sulfur through the scrubber in a short period of time and prevent excessive oxidation of sulfite ions within the liquor to sulfate during said interruption of flow.